This invention relates to a process for removing carbonate deposits from wells, especially from water wells which connect to water treatment facilities.
Groundwater is used as a source of water supply for much of the United States. Such water is typically drawn from underground deposits of unconsolidated granular materials, like sand and gravel, and from passages of sedimentary rocks, like limestone. Collection often involves pumping the water from the ground by means of a submersible or other pump through a well which extends from the Earth's surface down to the level of a water-bearing stratum or aquifer. The part of the well borehole above the aquifer is frequently covered with concrete and steel casing or similar structure to support the walls and prevent contamination of the water source by surface pollutants. The use of PVC has replaced steel as a casing material in the construction of most new water wells. Screens are sometimes fitted where the water flows into the well to prevent the inflow of loose materials from filling the well and clogging the system. Water wells may be public or private and serve to supply water for general household purposes, irrigation, livestock rearing, manufacturing, and other various uses.
After extraction from the well, the water is treated to bring it into conformity with required standards of purity. The water may be subjected to physical treatments to remove particulates and may also be exposed to chemicals to cause disinfection, oxidation and coagulation. The manner and degree of treatment will depend on the source of the water and its intended end use. For home use, attention must be paid to ensuring acceptable color, taste, odor and turbidity, as well as to the removal of pathogenic bacteria and harmful pollutants. Care must also be taken to assure a proper level of pH (acidity or alkalinity).
"Hard" waters, which are those having high calcium or magnesium content, are usually subjected to a softening procedure. One way of softening is to add lime and soda ash to cause calcium carbonate and magnesium hydroxide precipitation. This leaves the waters unstable, however, so that stabilization by recarbonation or other means is performed following treatment. Other softening methods include exposing the waters to zeolites (hydrated aluminum silicates) and passing the waters through ion exchange resins.
Where the groundwater supply is brackish, the treatment process typically includes a desalinization (desalting) step to render the saline water potable. Desalinization may be accomplished by distillation, electrodialysis, reverse osmosis, freeze-separation, hydration crystallization, or solvent-demineralization by ion exchange. In the reverse osmosis process, desalting is brought about by applying pressure to permit the passage of water through semipermeable membranes while impeding the passage of salt ions. (This is a reversal of the normal electrodialysis process in which electric current draws the salt ions through the membranes, leaving the desalted water.)
It is quite common, especially in deep water wells, for calcium and magnesium carbonate to be deposited in the water intake zone of the borehole by water flowing into the well. Although this phenomenon proceeds at a much greater rate, it is not unlike the deposition of calcium carbonate that results in the stalagmite and stalactite formations of limestone caves.
As groundwater moves through the host rock it is in contact with minerals, such as limestone (calcium carbonate), for a long time. Chemical dissolution takes place and the water moves together with dissolved mineral salts in chemical equilibrium toward the borehole. At the reservoir/well interface, the equilibrium is disrupted by the sudden change in pressure and precipitation of insoluble material (viz. calcium carbonate) results. The rate of carbonate deposition is related to water quality and to borehole entrance velocities during pumping.
With the passage of time, carbonate buildup in the well can begin to seriously interfere with water production. Water drawn from the well is replenished by new water from the cracks and fissures of the water-bearing stratum. If water is drawn from the well at a faster rate than the rate at which water from the stratum replaces it, the static water level in the well is lowered, and a condition known as "drawdown" exists. Severe drawdown can burn out the pump and shut down the treatment plant. Permeability reduction of the well caused by calcification, and particularly the build-up of calcium and magnesium carbonates in the region of the reservoir-well interface, is a leading cause of well drawdowns in deep water wells.
Well performance may be quantified in terms of well "specific capacity," which is the well discharge rate per unit of drawdown, expressed as gpm/ft. Carbonate removal is essential to restoring well specific capacity that has been unacceptably reduced by calcification.
Conventional techniques for removing carbonate from wells involve the pumping of strong concentrated acid (usually hydrochloric acid) into the well, allowing the acid to react with the formation for several hours, and then flushing the spent acid from the well. The results are not always effective due to the uneven distribution of acid within the well. Moreover, such procedures create the potential for serious injuries. The concentrated acid and its fumes can burn the skin and lungs. The acid can easily dissolve many types of pipe fittings, creating dangerous spills. Furthermore, fumes generated during treatment build up quickly and can cause large explosions if not properly vented. Accordingly, special crews and equipment are needed. Also, as a safety precaution, the fire department and/or emergency medical service are usually called to stand by to be able to flush spills and treat injuries.
The special crews and equipment, care in shipping and handling, and the attendant dangers makes the conventional carbonate removal process very costly, especially for rural community water supply organizations whose budget does not justify maintaining such resources in-house. It is not uncommon for such organizations to have to spend over $13,000 per well, two or more times per year.